When painting with a hand held spray gun, paint is fed to the gun either from a paint cup attached directly to the gun or from a remote paint cup connected through a paint hose to the gun. When the cup is attached to the gun, the paint may flow to a paint atomizing nozzle either through suction feed or through pressure feed established by pressurizing the cup. For remote cups, the paint is supplied under pressure. One general class of spray gun uses a flow of pressurized air for atomizing the paint as it is discharged from the nozzle. Within this class of spray gun, there are spray guns which use a low volume flow of high pressure compressed air and there are spray guns designed to operate from high volume low pressure (HVLP) air. HVLP spray guns operate with air at the nozzle at no more than 10 psig to meet regulatory requirements in jurisdictions such as California. The low volume high pressure air atomization spray guns often use the atomization air flow to create suction for causing paint to flow from a cup to the nozzle, where the air flow atomizes the paint. For an HVLP gun, there generally is insufficient air pressure at the nozzle to establish suction paint feed to the nozzle. Consequently, the paint cup must be pressurized to feed or at least assist feeding the paint to the nozzle. An HVLP spray gun may operate from a turbine which supplies the HVLP air, or it may operate from a high pressure air source. When the gun is designed to operate from a high pressure air source, one or more calibrated orifices are used to drop the air pressure to the desired low pressure for atomization and for pattern shaping and at the same time to increase the volume of the air flow.
When using high pressure air for atomization, the atomization air pressure may be between 40 psig and 100 psig (between 2.8 Kg/cm.sup.2 and 7 Kg/cm.sup.2), for example. At these pressures, a variation of 6 psig to 8 psig (0.42 Kg/cm.sup.2 to 0.56 Kg/cm.sup.2) in the paint feed pressure has little adverse effect on atomization. For an HVLP spray gun, the paint cup may be pressurized to, for example, to between 0 and 15 psig (between 0 and 1.05 Kg/cm.sup.2). The actual pressure used depends on the properties of the paint being atomized. For lower cup pressures, the paint cup may be pressurized by low pressure air from the nozzle applied through a check valve to the cup. For higher cup pressures, high pressure air must be used before it is dropped to the lower pressure required for atomization. The high pressure air is applied through a pressure reducing valve or orifice and a check valve to the cup. The check valve, which is mounted on the cup lid, prevents paint from entering the air passages and also maintains the cup pressure while the gun air flow is off. The check valve also may incorporate a pressure relief valve for manually venting the paint cup by pressing a valve button on top of the cup lid. Problems have occurred with prior art check valves used in low pressure systems when trying to control cup pressure to within 2 to 3 psig (0.14 to 0.21 Kg/cm.sup.2). Prior art check valves often required a pressure differential between the air supply and the cup of 3 to 4 psig (0.21 to 0.28 Kg/cm.sup.2) to open the valve and paint film accumulation on prior art valves has often increased the pressure differential to 6 to 8 psig (0.42 to 0.56 Kg/cm.sup.2) required to open the valve. Such a wide pressure differential makes it difficult to regulate the cup pressure to the desired accuracy. The prior art paint cup valves also are generally not easily removed from the cup lid for cleaning. Removal of many prior art valves requires tools for disassembly of the valve. The small valve parts are easily lost and may be difficult to reassemble.